The infrared spectrum covers a range of wavelengths longer than the visible wavelengths but shorter than microwave wavelengths. Visible wavelengths are generally regarded as between 0.4 and 0.75 micrometers. The near infrared wavelengths extend from 0.75 micrometers to 10 micrometers. The far infrared wavelengths cover the range from approximately 10 micrometers to 1 millimeter. The function of infrared detectors is to respond to energy of a wavelength within some particular portion of the infrared region.
Heated objects dissipate thermal energy having characteristic wavelengths within the infrared spectrum. Different levels of thermal energy, corresponding to different sources of heat, are characterized by the emission of signals within different portions of the infrared frequency spectrum. No single detector is uniformly efficient over the entire infrared frequency spectrum. Thus, detectors are selected in accordance with their sensitivity in the range of interest to the designer. Similarly, electronic circuitry that receives and processes the signals from the infrared detector must also be selected in view of the intended detection function.
A variety of different types of infrared detectors have been proposed in the art since the first crude infrared detector was constructed in the early 1800's. Virtually all contemporary infrared detectors are solid state devices constructed of materials that respond to infrared frequency energy in one of several ways. Thermal detectors respond to infrared frequency energy by absorbing that energy causing an increase in temperature of the detecting material. The increased temperature in turn causes some other property of the material, such as resistivity, to change. By measuring this change the infrared radiation is measured.
Photo-type detectors (e.g., photoconductive and photovoltaic detectors) absorb the infrared frequency energy directly into the electronic structure of the material, inducing an electronic transition which, in turn, leads to either a change in the electrical conductivity (photoconductors) or to the generation of an output voltage across the terminals of the detector (photovoltaic detectors). The precise change that is effected is a function of various factors including the particular detector material selected, the doping density of that material and the detector area.
By the late 1800's, infrared detectors had been developed that could detect the heat from an animal at one quarter of a mile. The introduction of focusing lenses constructed of materials transparent to infrared frequency energy, as well as advances in semiconductor materials and highly sensitive electronic circuity have advanced the performance of contemporary infrared detectors close to the ideal photon limit.
Current infrared detection systems incorporate arrays of large numbers of discrete, highly sensitive detector elements the outputs of which are connected to sophisticated processing circuitry. By rapidly analyzing the pattern and sequence of detector element excitations, the processing circuitry can identify and monitor sources of infrared radiation. Though the theoretical performance of such systems is satisfactory for many applications, it is difficult to actually construct structures that mate a million or more detector elements and associated circuitry in a reliable and practical manner. Consequently, practical applications for contemporary infrared detection systems have necessitated that further advances be made in areas such as miniaturization of the detector array and accompanying circuitry, minimization of noise intermixed with the electrical signal generated by the detector elements, and improvements in the reliability and standardized economical production of the detector array and accompanying circuitry.
Silicon is the substrate of choice for the fabrication of InAsSb infrared detectors because it facilitates the use of mature silicon-based device fabrication technology and also affords monolithic integration. However, contemporary InAsSb detectors formed upon silicon substrates exhibit poor detection performance, primarily due to a large number of defects which propagate from the silicon substrate into the InAsSb detector formed thereupon.
As those skilled in the art are aware, various different types of defects are known to exist within crystal structures, such as silicon substrates upon which infrared detector elements are formed. The different types of defects include point defects, line defects, and plane defects.
A point defect may be caused by a missing or extra atom within the crystal lattice. At the point where the missing or extra atom is located, the continuity of the crystal lattice is disrupted.
A line defect exists where a row of adjacent atoms in the crystal lattice is not bonded properly to an adjacent row of atoms. For example, one row may be shifted relative to the other such that each atom in a row bonds to an atom which is immediately adjacent the atom to which it would be bonded if no such defect existed.
A plane defect exists where two planar surfaces of crystalline material bond to one another in a manner which is not consistent with the adjacent crystalline structure. For example, each of the atoms of one planar surface may be shifted by the distance of approximately one atom spacing such that the atoms of one planar surface bond to atoms immediately adjacent to those to which they would normally be bonded, if no such defect existed, in a manner similar to that which occurs in a line defect. Thus, a plane defect is essentially a two dimensional line defect.
It is well known that crystal lattice defects propagate from one position to another within a crystal lattice over time. Such defect propagation may best be understood by way of an example utilizing a point defect. If one considers a point defect wherein an atom is missing from the crystal lattice structure, it is easy to understand that a hole effectively results at the site of the missing atom. Any of the atoms of the crystal lattice structure adjacent the hole may, over time, move so as to fill in the hole, thus leaving behind another hole where that atom had previously been located. The effect is the same as if the hole itself had moved. Thus, in this manner, the defect may tend to propagate to different locations throughout the crystal.
The silicon substrate upon which infrared detectors are formed is typically comprised of polycrystalline material wherein various regions of the substrate contain crystal lattices of different orientations which meet and interface with one another. At these interfaces the probability of a crystal lattice defect occurring is substantially increased. That is, when two crystalline regions of different orientations intersect, it is likely that a crystal lattice defect will occur at this location.
As such, the crystal lattice defect density within the silicon substrate is considerably greater than that of the single crystalline infrared detector elements. Thus, when single crystalline infrared detector elements are formed upon a polycrystalline substrate, it is likely that, over time, there will be greater propagation of crystalline defects from the substrate to the infrared detector elements than vice-versa. Since the performance of the infrared detector elements depends substantially upon the density of crystal lattice defects occurring therein, it is desirable to mitigate crystal lattice defect propagation from the substrate to the infrared detector elements.
It is not possible to isolate the detector elements from the substrate with many of the materials used in contemporary integrated circuit or detector fabrication, since it is necessary that any material to be disposed intermediate the detector elements and the substrate be compatible with the crystal structure of both the infrared detector elements and the substrate so as to afford monolithic construction. Thus, it is not feasible to utilize materials which substantially disrupt the crystal lattice structure of the infrared detector elements and the substrate. For example, it is not feasible to utilize vacuum deposited aluminum as a defect filter, since such a layer of aluminum, deposited upon a silicon substrate, would not provide the necessary crystal structure to seed subsequently deposited infrared detector element material.
As such, it is beneficial to provide a means for mitigating crystal lattice defect propagation from the silicon substrate or base layer upon which the infrared detector elements are formed into the InAsSb detector elements themselves.